1. Field of the Invention
The present invention concerns a method and a device for coding predictive filters for very low bit rate vocoders.
2. Description of the Prior Art
The best known of the methods of digitization of speech at low bit rate is the LPC10 or "linear predictive coding, order 10" method. In this method, the speech synthesis is achieved by the excitation of a filter through a periodic signal or a noise source, the function of this filter being to give the frequency spectrum of the signal a waveform close to that of the original speech signal.
The major part of the bit rate, which is 2400 bits per second, is devoted to the transmission of the coefficients of the filter. To this end, the binary train is cut up into 22.5 millisecond frames comprising 54 bits, 41 of which are used to adapt the transfer function of the filter.
A known method of bit rate reduction consists in compressing the 41 bit associated with a filter into 10 to 12 bits representing the number of a pre-defined filter, belonging to a dictionary of 2.sup.10 to 2.sup.12 different filters, this filter being the one that is closest to the original filter. This method has, however, a first major drawback which is that it calls for the construction of a dictionary of filters, the content of which is closely dependent on the set of filters used to form it by standard data processing techniques (clustering), so that this method is not perfectly suited to the real conditions of picking up sound. A second drawback of this method is that, to be applied, it requires a very large-sized memory to store the dictionary (2.sup.10 to 2.sup.12 packets of coefficients). Correlatively, the computation times become lengthy because the filter closest to the original filter has to searched for in the dictionary. Finally, this method does not enable the satisfactory reproduction of stable sounds. This is because, for a stationary sound, the LPC analysis in practice never selects the same filter twice in succession but successively chooses filters that are close but distinct in the dictionary.
Just as, in television, where the reconstruction of a color image depends essentially on the quality of the luminance signal and not on that of the chrominance signal which may consequently be transmitted with a lower definition, it appears, also in speech synthesis, that it is enough to reproduce only the contour of the energy of the vocal signal while its timbre (voicing, spectral shape) are less important for its reconstruction. Consequently, in known speech synthesis methods, the process of searching for spectra, based on the change in the minimum distance between the spectra of the original speech (of the speaker) and the synthetic speech is not wholly warranted.
For example, different examples of the sound "A" pronounced by different speakers or recorded under different conditions may have a high spectral distance but will always continue to be "A"s that cam be recognized as such and, if there is any ambiguity, in terms of a possibility of confusion with its neighboring sound, the listener can always make the correction from the context by himself. In fact, experience shows that in devoting no more than about 30 bits to the coefficients of the predictive filter instead of 41, the quality of restitution remains satisfactory even if a trained listener should perceive a slight difference among the synthesized sounds with the predictive coefficients defined on 30 or 41 bits. Furthermore, since the transmission is done at a distance, and since the intended listener is therefore not in a position to make out this difference, it would appear to be enough for the listener to be capable of understanding the synthesized sound accurately.
It would also appear to be important that, in the stable parts of the signal (the vowels), the predictive filter should remain stable and be as close as possible to the original predictive filter. By contrast, in the unstable parts (such as transitions or unvoiced sound), the transmitted predictor does not need to be a faithful copy of the original predictor.
It is an aim of the invention to overcome the above-mentioned drawbacks.